Demagnifying projection exposure using ultraviolet light has long been performed as a lithographic method for manufacturing micro-semiconductor devices such as semiconductor memories and logic circuits. The smallest dimensions that can be transferred by demagnifying projection exposure are proportional to the wavelength of the light used in transfer and inversely proportional to the numerical aperture of the projection optics. In order to transfer microcircuit patterns, therefore, shorter wavelengths are being adopted for the light used. For example, the wavelength of ultraviolet light used has become progressively shorter, i.e., 365 nm using mercury lamp i-rays, 248 nm using a KrF excimer laser and 193 nm using an ArF excimer laser.
However, semiconductor devices are becoming smaller and smaller at a rapid pace and lithography using ultraviolet light imposes limitations on demagnifying projection exposure. Accordingly, in order to perform the lithography of very fine circuit patterns of less than 1 μm in an efficient manner, a demagnifying projection exposure apparatus using extreme ultraviolet (EUV) light having a wavelength on the order of 10 to 15 nm, which is much shorter than that of ultraviolet light, has been developed.
Because absorption of light by substances is extremely high in the EUV region, lens optics that utilize the refraction of light, as employed with visible light and ultraviolet light, are impractical and, hence, use is made of reflective optical systems in exposure apparatus that rely upon EUV light. These reflective optical systems employ a reflective reticle obtained by forming the pattern, which is to be transferred, on a mirror using an absorbing body.
A multilayer mirror and an oblique-incidence full-reflection mirror are examples of reflectance-type optical elements for constructing an exposure apparatus that relies upon EUV light. In the EUV region, the real part of the index of refraction is much smaller than unity and, as a result, total reflection occurs if use is made of oblique incidence in which the EUV light just barely impinges upon the mirror surface. Usually, a high reflectivity of twenty or thirty percent or more is obtained by oblique incidence of within several degrees measured from the surface. However, because such oblique incidence diminishes degree of freedom in terms of optical design, it is difficult to use an oblique-incidence full-reflection mirror in a projection optical system.
A multilayer mirror obtained by building up alternating layers of two types of substances having different optical constants is used as a mirror for EUV light employed at an angle of incidence close to that of direct incidence. For example, molybdenum and silicon are formed in alternating layers on the surface of a glass substrate polished to have a highly precise surface shape. The layer thicknesses of the molybdenum and silicon are, e.g., 0.2 nm and 0.5 nm, respectively, and the number of layers is 20 each. The combined thickness of two layers of the different substances is referred to as the “film cycle”. In the above example, the film cycle is 0.2 nm+0.5 nm=0.7 nm.
When EUV light impinges upon such a multilayer mirror, EUV light of a specific wavelength is reflected. Only EUV light of a narrow bandwidth centered on a wavelength X that satisfies the relationship of Bragg's equation2×d×sin θ=λ  (1)where λ represents the wavelength of the EUV light and d the film cycle, will be reflected efficiently. The bandwidth in this case is 0.6 to 1 nm.
The reflectivity of the reflected EUV light is 0.7 at most, and the unreflected EUV light is absorbed in the multilayer films or in the substrate. Most of this energy is given off as heat.
Since a multilayer mirror exhibits more loss of light than a mirror for visible light, it is necessary to hold the number of mirrors to the minimum. In order to realize a broad exposure area using a small number of mirrors, use is made of a method (scanning exposure) in which a large area is transferred by causing a reticle and a wafer to perform scanning using fine arcuate areas (ring fields) spaced apart from the optical axis at fixed distances.
FIG. 8 is a schematic view illustrating a demagnifying projection exposure apparatus that employs EUV light according to an example of the prior art. This apparatus includes an EUV light source, an illuminating optical system, a reflecting-type reticle, a projection optical system, a reticle stage, a wafer stage, an alignment optical system and a vacuum system.
By way of example, a laser plasma light source is used as the EUV light source. Specifically, a target material placed in a vacuum vessel is irradiated with high-intensity pulsed laser light from a light source 801, a high-temperature plasma is produced and EUV light having a wavelength of, for example, 13 nm that emanates from the plasma is utilized as the EUV light source. A thin film of metal, an inert gas or a droplet is used as the target material, which is supplied by a target supply unit 802, and is fed into the vacuum vessel by means such as a gas jet. In order to increase the average intensity of the EUV light emitted, the pulsed laser should have a high repetition frequency and the apparatus should be operated at a repetition frequency of several kilohertz.
The illuminating optical system comprises a plurality of multilayer or oblique-incidence mirrors (803, 804, 805) and an optical integrator 806, etc. A condensing lens 803 constituting a first stage functions to collect EUV light that emanates from the laser plasma substantially isotropically. The optical integrator 806 functions to illuminate a reticle 814 uniformly using a prescribed numerical aperture. An aperture for limiting to a circular arc the area of the reticle surface that is illuminated is provided at a conjugate point with respect to the reticle 814 disposed in the illuminating optical system.
The projection optical system uses a plurality of mirrors (808 to 811). Though using a small number of mirrors allows EUV light to be utilized very efficiently, this makes it difficult to correct for aberration. The number of mirrors needed to correct for aberration is four to six. The shapes of the reflecting surfaces of the mirrors are convex or concave spherical or non-spherical. The numerical aperture NA is 0.1 to 0.3. To fabricate the mirror, use is made of a substrate consisting of a material, such as glass having a low coefficient of expansion or silicon carbide, that exhibits a high rigidity and hardness and a small coefficient of expansion, the substrate is formed to have a reflecting surface of a predetermined shape by grinding and polishing, and multilayer films such as molybdenum and silicon are formed on the reflecting surface. In a case wherein the angle of incidence is not constant owing to the location of the layer in the mirror surface, the wavelength of the EUV light, the reflectivity of which rises depending upon the location, shifts, if use is made of multilayer films having a fixed film cycle, as is evident from Bragg's equation cited above. Accordingly, it is required that a film-cycle distribution be provided in such a manner that EUV light of the same wavelength will be reflected efficiently at the mirror surface.
A reticle stage 812 and a wafer stage 813 have a mechanism in which scanning is performed synchronously at a speed ratio proportional to the reducing magnification. Let X represent the scanning direction in the plane of the reticle or wafer, Y the direction perpendicular to the X direction, and Z the direction perpendicular to the plane of the reticle or wafer.
The reticle 814 is held by a reticle chuck 815 on the reticle stage 812. The reticle stage 812 has a mechanism for high-speed movement in the X direction. Further, the reticle stage 812 has a fine-movement mechanism for fine movement in the X, Y and Z directions and for fine rotation about these axes, thus making it possible to position the reticle. The position and attitude of the reticle stage are measured by laser interferometers (not shown) and are controlled based upon the results of measurement.
A wafer 816 is held by a wafer chuck 817 on the wafer stage 813. Like the reticle stage, the wafer stage 813 has a mechanism for high-speed movement in the X direction. Further, the wafer stage 813 has a fine-movement mechanism for fine movement in the X, Y and Z directions and for fine rotation about these axes, thus making it possible to position the wafer. The position and attitude of the wafer stage are measured by laser interferometers (not shown) and are controlled based upon the results of measurement.
Consider an arrangement in which an alignment detection system (818, 819) is implemented by an off-axis bright-field illuminated image processing system similar to that of, e.g., an ArF exposure apparatus, and wafer alignment is carried out while a predetermined baseline amount is maintained.
Further, the focus position along the Z axis on the wafer surface is measured by a focus-position detecting optical system 820, and the position and angle of the wafer stage are controlled. During exposure, therefore, the surface of the wafer is always maintained at the position at which the image is formed by the projection optical system.
When a single scan of exposure of the wafer ends, the wafer stage is stepped in the X and Y directions to move the stage to the starting position of the next exposure scan, then the reticle stage and wafer stage are again scanned synchronously in the X direction at a speed ratio that is proportional to the reducing magnification of the projection optical system. Thus, an operation for synchronously scanning the reticle and wafer in a state in which the demagnified projection image of the reticle is formed on the wafer is repeated (by a step-and-scan operation). The transfer pattern of the reticle is thus transferred to the entire surface of the wafer.
FIG. 9 is a diagram showing an arrangement for reticle alignment according to the prior art. In the specification of this application, “reticle” and “mask” will be referred to generically as a “reticle”.
Reticle alignment involves achieving relative alignment between a reticle reference mark 60, which has been positioned accurately on the apparatus proper, and a reticle alignment mark 4 situated on a reticle 1. In FIG. 9, alignment light having a wavelength different from that of the exposing light is reflected on the side of the reticle side by a prism 80 and illuminates the reticle reference mark 60 and the reticle alignment mark 4. Light from the marks has the direction of its optical path changed by a deflecting mirror 70 and is directed toward an image sensing device 10 via an optical system 40 so that the images of both the reticle reference mark 60 and reticle alignment mark 4 are formed on the image sensing device 10. Owing to the positional relationship between the images of the two marks, the amount of positional deviation with respect to the reticle 1 is calculated. Based upon the result of calculation, the reticle stage 812 is driven by a drive unit (not shown) to achieve alignment between the reticle alignment mark 4 and reticle reference mark 60. Performing this alignment completes the aligning of the reticle and exposure apparatus proper.
However, when reticle alignment in an X-ray demagnifying projection exposure apparatus (EUVL) is considered, the fact that the reticle used is a reflective reticle means that it is impossible to detect the mark images of both the reticle reference mark and reticle alignment mark simultaneously by “transmitting” the images.
Further, since the reflective reticle and multilayer mirror are optimized so as furnish a high reflectivity with EUV light, there is the possibility that a sufficient reflectivity will not be obtained for the alignment light, which is non-exposing light. In other words, in a case wherein consideration is given to so-called TTL (Through-the-Lens) alignment performed via a reflective reticle and a multilayer mirror using non-exposing light, there is the possibility that owing to alignment between the reticle alignment mark on the reflective reticle and the mark on the wafer, the image detection signal will decline because the the reticle alignment mark exhibits low reflectivity with respect to alignment light.
In the ordinary projection exposure apparatus, a method of aligning the reticle and wafer via a projection lens is referred to as TTL alignment. In an EUV exposure apparatus, however, the projection optical system is constituted not by lenses but by the multilayer-mirror optical system. It is difficult, therefore, to refer to this scheme as a TTL scheme. However, for the sake of simplifying the description, an alignment system that uses the intervention of a multilayer-mirror optical system will also be defined as being a TTL alignment scheme in this specification.